The former Hewlett-Packard chief executive mocked Mr Trump’s assertion that he is the Republican’s “presumptive nominee” during her speech at the California Republican Party’s convention.

Mr Cruz is mathematically eliminated from becoming the party’s presidential nominee unless Mr Trump fails to reach the target of 1,237 delegates needed to clinch the nomination. This would force a contested Republican National Convention in July where Mr Cruz can fight for the prize in a second ballot.

“The 30-yard-line ain’t a touchdown,” she said. “The 20-yard-line ain’t a touchdown. The 5-yard-line ain’t a touchdown. It ain’t a touchdown until it’s a touchdown.”

She added: “I’m sorry, you cannot just throw an ‘R’ on your jersey and say you believe what our party stands for.”

It was something of a homecoming for Ms Fiorina, who grew up in the Bay Area. After leaving her position at Hewlett-Packard, she unsuccessfully ran against Democratic Senator Barbara Boxer in 2010.

She flew in after a day of stumping for Mr Cruz in Indiana, a state he must win if he is to have any chance of stopping Mr Trump.

California is assuming importance this primary season. The state’s 172 delegates up for grabs on June 7 may decide whether Donald Trump becomes the party’s nominee or if Mr Cruz still has a chance on a second ballot.

Mr Trump’s opponents are hopeful Ms Fiorina can help Mr Cruz in the state’s primary.

“She campaigned in this state very well, she won with these voters,” said Rob Stutzman, a Republican operative helping with efforts to defeat Mr Trump in the primary.

“She was a good Republican candidate here. She’s the perfect surrogate for Cruz.”

Ms Fiorina won a bruising Republican primary in 2010, outflanking her opponents from the right to become the party’s nominee to take on Ms Boxer, a liberal who was viewed as vulnerable in 2010.

But Ms Fiorina was comprehensively beaten in the general election, losing by one million votes after being hammered for her stewardship of Hewlett-Packard and role in outsourcing jobs overseas.

State party chairman Jim Brulte said Ms Fiorina was the first presidential candidate to respond to his invitation to appear at the state party convention that he issued to the entire presidential field in January, when she was still a contender for the top office. “That’s probably because I sent it to her personal email account,” he said.

Mr Cruz spoke to the convention on Saturday and highlighted his running mate, saying she would be the first Californian on a national ticket since Ronald Reagan.

But Ms Fiorina disappointed some Republicans in California. Republican strategist Matt Rexroad is leaning towards Mr Cruz but said the selection of Ms Fiorina makes him question whether he will support the Texan senator.

Mr Rexroad admitted Ms Fiorina will be popular among the party faithful at the convention. But that group is a tiny minority of Republicans who will vote on June 7. “That isn’t necessarily reflective of the electorate,” he said.